The present invention relates generally to electrical discharge machining (EDM), and, more specifically, to EDM drilling.
Electrical discharge machining is a process in which a cathodic electrode is positioned atop an electrically conducting workpiece, and a liquid dielectric is channeled therebetween. Electrical current passes between the electrode and workpiece and locally erodes the workpiece for machining thereof. In a typical application, the electrode may be used for drilling a hole of any desired shape in the workpiece.
For example, many gas turbine engine components include small holes therein through which cooling air is channeled during operation. The holes are small in diameter and typically range from 10–80 mils (0.2–2.0 mm), and require a slightly smaller diameter EDM electrode.
The narrow electrodes are consumed during machining, and are therefore initially relatively long in length which typically ranges from about 12–16 inches (30–41 cm) for obtaining a useful life during drilling.
Furthermore, the electrodes are typically tubular for channeling the liquid dielectric therethrough during operation. Accordingly, the hollow, slender electrodes are relatively flexible in bending along their longitudinal axes. Such flexibility is typically not desirable since it adversely affects the accuracy and repeatability of EDM drilling.
More specifically, the electrode tip must be accurately maintained at a small clearance gap of about 1 mil (0.025 mm) with the workpiece to effect suitable electrical discharge machining without experiencing an undesirable electrical short circuit therewith.
Accordingly, the electrode tip is typically mounted through a lower guide that accurately maintains a side clearance around the electrode as it drills through the workpiece. And, the opposite or top end of the electrode is held in a conventional electrode holder which is effective for translating the electrode downwardly toward the workpiece during operation, and for maintaining the small clearance gap vertically therebetween.
In this way, the lower tip guide and the upper electrode holder accurately support both ends of the electrode for maintaining the desired gap both laterally around the electrode tip and vertically between the tip and the workpiece during the EDM operation.
However, the electrodes have a maximum length at the beginning of the drilling operation, with corresponding maximum flexibility, and are consumed during drilling which decreases their length and flexibility correspondingly.
Electrode flexibility becomes one problem due to the substantially high pressure of the dielectric channeled therethrough. Dielectric pressures up to about 50 atmospheres are conventional and produce a jet of dielectric discharge from the electrode tip against the workpiece as a hole is drilled.
The electrode correspondingly experiences a reaction force which acts in compression therethrough. Since the electrode is a slender rod or column, it is subject to compressive buckling loads which can cause lateral deflection of the electrode that correspondingly shortens its effective length and withdraws the electrode tip slightly away from the workpiece, and affects EDM performance.
Although tubular electrodes are nevertheless efficient in forming cylindrical holes in turbine components, improved turbine cooling can be obtained with more complex hole configurations. For example, turbine nozzle vanes and rotor blades have thin walled airfoils typically including a multitude of film cooling holes extending therethrough.
The airfoils have elaborate internal cooling circuits for cooling the inside thereof during operation in the hot combustion flowpath, with the spent internal cooling air being discharged through the film cooling holes to form thin layers of air on the external surface of the airfoil for providing thermal insulation against the hot combustion gases. Since the film cooling air is discharged under pressure, small jets of air are developed and adversely affect the formation of a continuous external air film.
A significant improvement in film cooling design includes a diffusion hole typically having a cylindrical inlet and a diverging outlet that diffuses the jet of cooling air and correspondingly reduces the discharge velocity thereof. Diffusion film cooling holes improve the performance of the external thermally insulating air film.
However, diffusion holes have complex configurations and cannot be formed by the simple tubular EDM electrode commonly used for cylindrical film cooling holes. Instead, a specifically configured EDM electrode is required for machining the complex diffusion hole, but EDM wear of the electrode becomes more of a problem. As the complex electrode wears it is not conveniently repairable, and must be replaced in whole.
A typical form of the diffusion electrode is a comb having a row of identical fingers configured for simultaneously forming the small inlet and diverging outlet of each diffusion whole.
The EDM comb electrodes may be economically manufactured in a stamping process, but nevertheless the combs wear during operation and must be replaced in whole. Furthermore, the comb requires parallel fingers which in turn requires identical orientation of the resulting film cooling holes.
However, modern gas turbine engine design may include a multitude of film cooling holes having different configurations around the external surface of the airfoils between the leading and trailing edges thereof, and it may be desirable to vary the configuration of the holes along both the radial or longitudinal span of the airfoils and axially along the chords.
Conventional EDM electrodes and apparatus therefor lack the versatility to economically drill or machine a multitude of film cooling holes having different orientations and different diffusion configurations.
Accordingly, it is desired to provide an improved EDM apparatus and process for drilling complex diffusion film cooling holes in workpieces such as turbine airfoils.